Organic semiconductor materials are interesting candidates for large area electronic applications, such as smart cards, information tags, flat panel displays or large area sensors. Up to now polyacenes (e.g., naphthalene, anthracene, tetracene and pentacene) have demonstrated the best performance of organic materials in terms of mobility, speed and on/off ratio. These organic materials exhibit a high tendency to form highly ordered films, which can be poly or single crystalline depending on the deposition and substrate conditions.
Different methods have been proposed to realize organic thin film transistors (TFTs) with good electronic properties. For example, vapor phase deposition, thermal deposition, and solution based (spin, dip casting, ink jet printing technologies) fabrication techniques have been explored by various entities. In the case of vapor phase deposition, the organic single crystals are grown in a stream of gas (carrier gas), which transfers the vaporized material from the source to the substrate. The realized transistor structures exhibit high mobilities of 2-3 cm2/Vs. However, this deposition method cannot be applied to large areas, which are of interest for low cost electronics. Solution based processes are an alternative to the vapor phase deposition. Spinning and dip casting on one hand and ink jet printing on the other hand are promising methods to realize low cost electronics on low cost substrates. In the case of direct printing of the semiconductor material, the organic film can be prepared and patterned in one step. However, the mobility of the solution based polyacene TFTs is at least one order of magnitude reduced due to the structural disorder of the film (0.1 cm2/Vs).
Thermal evaporation is the third fabrication method for producing organic TFTs, and the one used most widely. In this case, the organic material is vaporized under high vacuum conditions and deposited on a heated substrate. The mobility ranges from 0.2 cm2/Vs to 1.4 cm2/Vs. Thermal evaporation of organic films is a very promising method for large area electronics, because the fabrication process combines the advantage of good material properties with the ability to realize transistor devices on large area substrates.
The performance of organic TFTs, specifically the carrier mobility, depends highly on the structural order of the organic film, which is determined both during formation and by subsequent processing. Regardless of the method used to form the organic film, the structural order of an organic film depends strongly on the surface properties of the underlying material on which the organic material is formed. That is, the surface morphology and the chemistry of the underlying material have an influence on the subsequent growth of the organic film. Furthermore, it is difficult to pattern the organic films without affecting the electronic properties. Therefore, bottom-gate (inverted) transistor arrangements, in which the organic semiconductor film is formed on a dielectric layer that overlies a gate electrode, are the preferred organic TFT design. Further, conventional bottom-gate organic TFTs are typically fabricated on thermal oxide dielectric layers grown on monocrystalline silicon substrates. Such thermal oxides provide superior surface properties that have been used to produce organic TFTs exhibiting high carrier mobility. Therefore, most organic semiconductor devices are currently formed on thermal oxide grown on silicon wafers.
A problem with utilizing organic semiconductor materials for large area electronic applications is that silicon wafers are not practical for large area applications, and large area compatible inorganic dielectrics such as PECVD silicon nitride and silicon oxide are generally found to produce inferior organic TFTs. Typical inexpensive large area substrates (e.g., plastic foil or glass) do not support the growth of thermal oxide, so dielectric materials must be deposited using, for example, Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques. These deposited dielectric materials (e.g., silicon nitride, silicon oxide, or silicon oxy-nitride) are known good dielectric materials for the formation of amorphous silicon and polysilicon transistors, but have been found to be unsuitable for the formation of highly ordered organic films. Therefore, organic semiconductor devices produced on inexpensive large area substrates have inferior performance characteristics when compared to organic semiconductor devices formed on thermally grown oxides.
What is needed is a method for producing organic semiconductor materials on deposited dielectrics that provide similar performance characteristics to organic semiconductor materials formed on thermally grown oxides.